JPS61124855A - Method for leading out image information using nuclear magnetic resonance - Google Patents

Abstract

PURPOSE:To obtain image information reduced in the deterioration of an image by an artifact, by performing the collection of data so as not to stepwise change the integrated value of the intensity of an inclined magnetic field and a magnetic field applying time and subsequently rearranging the data to perform the processing thereof. CONSTITUTION:In replacing an inclined magnetic field (GZ) pulse for performing phase encoding, when a data collection timing interval is set to Tr=1sec, the cycle of the displacement of an abdominal part is Tp=4sec and, therefore, four displacement states are taken at each cycle and a rearranged data system is divided into four groups and each group is set to one of four displacement states. As shown by F, the GZ pulse is changed at every 64pi in a range from -128pi to 127pi. Thus obtained nuclear magnetic resonance data system is rear ranged as shown by G so as gradually change the integration value of GZ and the time before sampling the same. At this time, the displacement component by respiration contained in this data system is divided into four groups as shown by M and becomes almost equivalent to respiration performed once for 256sec.

Description

DETAILED DESCRIPTION OF THE INVENTION "Field of Industrial Application" This invention relates to a method for deriving image information from a subject using nuclear magnetic resonance signals.

"Prior art" An image forming apparatus using nuclear magnetic resonance (hereinafter referred to as NMR) is disclosed in, for example, Japanese Unexamined Patent Publication No. 1983-6347, magazine "Image Diagnosis" published by Shujunsha in 1988, Vol. 12, No. 1, 2G- This is shown on page 42, etc.

In an image forming apparatus using the NMR method, for example, as shown in FIG. G! is generated, and the subject 13 is placed in the space where these static magnetic fields and gradient magnetic fields are produced.
will be arranged.

Note that the three gradient magnetic fields only need to have the gradient directions of their magnetic field strengths intersecting each other, and do not necessarily need to be orthogonal. In this way, spatial information from the subject 13 can be discriminated.

A pulse current is applied to the transmitting/receiving coil 14 to apply a high frequency magnetic field to the subject 13 in a direction perpendicular to the Z axis.

The NMR signal from the subject 13 generated at that time is the coil 1
4, and the received output is sent to the high frequency signal transmitting/receiving section 15.
The detection output is amplified and detected at A.
The signal is converted into a digital signal by a D converter 16, and then manually inputted to a signal processing device 17 consisting of an electronic computer.

By the way, NMR relies on the fact that when certain atomic nuclei are placed in a magnetic field, they precess around the direction of the applied magnetic field at a frequency proportional to the strength of the magnetic field. This frequency is known as the Larmor frequency and is given by ωo =rH. However, T is the gyromagnetic ratio of the atomic nucleus, and Ha is the strength of the magnetic field. When a magnetic field whose strength varies along a certain direction, a so-called gradient magnetic field, is applied, atomic nuclei (hereinafter referred to as nuclear spins) at different positions in that direction precess at different frequencies.

Using this property, the position information of the nuclear spin, that is, the object 1
There are roughly two methods for obtaining information indicating which position among the three. One method is to apply a gradient magnetic field to a certain axis, keep its intensity constant over time, collect NMR signal data during that time, and use frequency encoding to directly detect the Maki spins along this axis. This is a method of obtaining location information. The other method is to apply a gradient magnetic field to a certain axis and sequentially change the time-integrated value of its intensity, collect NMR signal data, and use phase encoding to determine the position of the spins along this axis. It is a way to obtain information. In either case, several methods can be considered to obtain position information from signal data, but Fourier transform is most commonly used today, so the following explanation uses this as an example. In the case of Fourier transform, the data is basically converted from the time axis to the frequency axis, so the data before measurement processing must be a temporally ordered signal sequence to be meaningful. ,
In the case of frequency encoding, the gradient magnetic field strength applied to a certain axis is held constant during signal data collection, and in the case of phase encoding, the time-integrated value of the gradient magnetic field strength applied to a certain axis is sequentially maintained. Changes are being made to. That is, in both frequency encoding and phase encoding, the integral value of the strength of the gradient magnetic field up to the sampling point and the time of applying the magnetic field is made to change successively for each sampling.

In such data processing, by directly Fourier transforming the obtained signal data series, the position information of the nuclear spin along a certain axis can be obtained from the information obtained from the NMR signal, such as the density of the spear spin, It is easy to understand that spatial distributions such as relaxation times can be obtained. The arithmetic processing unit 17a in the signal processing device 17 performs arithmetic processing to obtain such spatial distribution, that is, image information, and the obtained image information is displayed as an image on the display device.

The spin warp method is a method for obtaining NMR signals in order to obtain positional information of tomb spins (for example, see the above magazine @).
To obtain a two-dimensional image on the After this excitation, a read gradient magnetic field G is applied in the X direction during period 5, and the NMR signal at this time is received, for example, 256 times and frequency encoded. In other words, by performing a 256-point Fourier transform, information on the position of the nuclear spin along the X-axis (the frequency represents the position on the X-axis, and the amplitude at that position represents the vector sum of the nuclear spin in two directions) is obtained. . Between said selective excitation and reading, i.e. period 4
A gradient magnetic field G1 in the Z direction is applied in a pulsed manner, and its amplitude is sequentially changed at equal intervals (in steps) each time.

For example, if this amplitude change is set to 256 steps, the image field is -a<mx<a, -a<wz<a, and the phase of the nuclear spin of zs"a is the n-th phase of the nuclear spin of 2=0. When transmitting and receiving high-frequency pulses, a magnetic field G1 of such intensity as to advance (n - 129)π is applied.

In this way, 256 G, pulses are each 256
The response is maximized at each position in the Z direction for each spatial frequency (corresponding to the position on the X axis). The signal obtained in this way is subjected to two-dimensional Fourier transform to
An image of 56×256 pixels can be obtained. G here
, the width of the G pulse may be successively changed without successively (stepwise) changing the amplitude of the G pulse, and further, the product of both the amplitude and the width of the G2 pulse may be successively changed. To obtain the spatial distribution of the spin-lattice relaxation time, which may vary, in Figure 2, a 180° pulse or AFP (adiabatic fast passage) is applied during period 1 to invert the nuclear spins and obtain the average relaxation time. It is sufficient to wait a certain amount of time (period 2) and then move on to period 3.

The second scan condition determination unit 17b in the signal processing device 17
A signal for controlling the generation of the pulse sequence shown in the figure is sent to the gradient magnetic field generating means 12 and the high frequency signal transmitting/receiving section 15. The signal processing device 17 generally includes an electronic computer, which performs processing control. FIG. 1 functionally shows a part of the functions of the signal processing device 17.

``Problem IIjY that the invention seeks to solve'' The subject 13 from which point of view information is to be obtained is, for example, a living body and is not necessarily stationary, or the strength of the generated magnetic field is unstable over time. In reality, there are elements that change while collecting NMR signals. For this reason, the frequency encoding and phase encoding of the obtained signal are not necessarily performed ideally, and the image formed by calculation processing based on this has artifacts C corresponding to the change pattern of the above-mentioned elements. This results in parts that differ from the original image (so-called false images), deteriorating both awards. In particular, with current technology, it takes several minutes to collect the data necessary for image formation using nuclear NMR signals, which is a long time compared to other image forming devices using X-rays, ultrasound, etc. For, NM
In the case of an R image forming apparatus, the influence of the above-mentioned fluctuation is significant and cannot be ignored.

For this reason, conventionally, changes in the above elements are detected,
NMR signals were collected only at the times when the displacements became the same value, and changes in the above elements were used to minimize the amount of change included in the data series for calculation processing. When considering image formation in the heart, the displacement of the beating heart is detected by an electrocardiograph, and NMR signal data is always collected at a constant timing with respect to the heartbeat, thereby providing data for calculation processing. If you look at the series, you will get an image that looks as if the heart had stopped opening for several minutes while the data was being collected, except for some errors.

This is called an electrocardiogram-gated scan. According to this method, it is certainly possible to minimize artifacts due to fluctuations, but the timing interval Tr for signal data collection is determined depending on the temporal pattern of fluctuations in the object to be examined, and nuclear magnetic resonance images There is a drawback that the data collection timing interval (sampling interval) Tr, which should originally be a very important parameter, cannot be specified arbitrarily in forming the data collection method. Furthermore, if the object to be examined is a part of the living body that involves respiratory movements, the Tr by the above method will be very long, about 4 seconds, and the time required for data collection will be several tens of minutes. The result is that This is not only inefficient, but it is impossible to avoid the disadvantage that it increases the risk of disaster for the subject, such as when taking a close-up image of the human body.

For example, when obtaining an image of the abdomen of a human body, the abdomen of the human body is generally vibrated and displaced due to breathing. The period Tp is about 4 seconds. This displacement is shown in FIG.
When measuring by sequentially changing the pulse, the signal data collection timing interval Tr is 1 second, and the above (n-129)
If the amplitude (or pulse width) of the Gz pulse that advances tt is expressed by two phase differences, give the Gz pulse shown in Figure 3B at each data acquisition timing. As shown in Figure 3C, a change pattern based on displacement due to breathing is included. If this data is subjected to Fourier transform as it is, artifacts based on this change pattern will occur and the image quality will deteriorate.

Figure 4 shows the results of simulating this situation.
In FIG. 4, the lower thick line 21 is a part of the subject,
This vibrates in the Z-axis direction due to breathing, and in the left-right direction in the lower part of Figure 4, with a period of 4 seconds.

After Fourier transforming the NMR signal sampled at Tr = 1 second in the direction of the frequency encoding axis, the signal viewed in the direction of the phase encoding axis becomes a curve 22 shown in the upper part of Fig. 4, which shows the fluctuation elements included in this song 1s22. The temporal change pattern is the upper curve 23 in Figure 4,
This curve 23 has 64 vibrations. Tmari Tr
= To obtain data of 256 x 256 pixels in 1 second, 25
It takes 6 seconds, during which time the abdomen increases by 25
It can be seen that the vibrational displacement is 6+4-64 times and this is included in the collected data. Curve 22 which is this collected data
When it is Fourier transformed, it becomes the shape shown in the lower part of Fig. 4 with diagonal lines. This diagonal shape is the shape in the Z-axis direction shown as the measurement result, and the difference between this and the shape of the actual image 1421 is an artifact, and in particular, images 24 and 25, which actually do not exist at all, appear greatly.

If the conventional synchronous scanning method is applied to the example in FIG. Data will be collected every time the displacement becomes the vibration center, and a gradient magnetic field of 0 is applied once every period Tp-4 seconds of the displacement vibration to collect data.At this time, the data obtained will be The component due to respiratory vibration before processing is constant as shown in Figure 3E, and is not affected by respiratory vibration.However, in this method, the data collection timing interval Tr cannot be freely selected; Since the respiratory cycle must be set to Tp - 4 seconds, it takes 4 seconds x 256 seconds to collect all the data required for image formation.
- It takes a long time of 1024 seconds.

The purpose of this invention is to enable the data collection timing interval Tr to be freely selected, and to perform nuclear magnetic resonance in a short data collection time without being affected by the presence of various variable factors, that is, with less deterioration of image quality due to artifacts. The object of the present invention is to provide a method for deriving image information using the present invention.

"Means for Solving the Problem" According to the present invention, the integral value of the strength of a gradient magnetic field with respect to one coordinate axis and the time of application of the magnetic field up to the data acquisition point is collected so that it does not change sequentially (stepwise). After that, the captured data is rearranged so that the integral value changes sequentially, and then processing for obtaining spatial distribution information is performed.

In other words, changes due to the heart movement of the subject, movement due to breathing, fluctuations in the magnetic field, etc. can be predicted in advance, or the changes can be detected just before NMR@ collection, and the sampling can be adjusted accordingly. The data obtained in this way is rearranged in temporal order so that the integral values are sequential, and the rearranged data series is subjected to, for example, Fourier transform. The non-sequential sampling is performed to obtain spatial distribution information, but the non-sequential sampling is performed so that the change component included in the rearranged data series is not noticeable in the data series.

"Example 1" For example, when changing the gradient magnetic field Gz pulse for performing phase 2 encoding in FIG. 3, if the data collection timing interval Trm is 1 second, the period of abdominal displacement Tp-
Since it is 4 seconds, four displacement states are taken in each cycle, and the changed data series is arranged into four groups, and each group is one of the four displacement states.0 For example, as shown in Figure 3F. In the first data collection, −128g was given as the Gz pulse, and in the second time, -64π was given, ′
The 33rd time is 0π, the 4th time is 64π, and so on.
G2 pulses are given by changing every 4π, then in the next 5th time, it returns to -127π, and in the 6th time, it gives -63π, etc., and so on, the G2 pulse is applied every 64π in the range from -128π to 127π. change.

In the case of 0-phase encoding, in which the NMR signal data series obtained in this way is rearranged so that the integral value between Gz and the time until sampling changes sequentially, the NMR signal data series obtained in this way is rearranged as shown in Figure 3G. , Gz pulse is -128,
The data collected at -127ff, -126π... are arranged in that order.

At this time, the displacement components due to respiration included in this data series are divided into four groups as shown in Figure H of 1JIJ3. That is, the Gz pulse is 1128π to -65π
The interval includes the center component of the displacement due to breathing, and the interval of the Gz pulse from 164π to -π includes the center component of the displacement due to Tp/
The displacement component at 4 is included, and the Gz pulse is θ ~ 63π
The section includes a displacement component at T p /2 from the center of displacement, and the section where the Gz pulse is 64π to 127π is a displacement component at 37p/4 from the center of displacement. In this data series, the Gz pulse is 1128π to -65π,
The effect of displacement due to breathing is exactly the same in each of the four sections (groups) -64g~-π, O~63π, and 64~127π, so the rate of fluctuation due to breathing is small for the data series as a whole. Therefore, it can be said that it is not affected by breathing. In other words, in this example, it is almost equivalent to taking one breath in 256 seconds.

FIG. 5 shows the result of simulating the collected data subjected to such processing in the same manner as in FIG. 4, and parts corresponding to those in FIG. 4 are given the same reference numerals. In this case, the fluctuation component included in the rearranged data series becomes the curve 23 in the upper row of FIG. 5, and the artifacts 24 and 25 in FIG. It can be seen that this has been greatly improved. Moreover, in the case of this method, the signal data collection timing interval Tr can be selected freely to some extent, and the time required for signal data collection can be significantly shortened. The measurement time is reduced to one fourth compared to the scanning method. In addition, in FIG. 1, the NMR signal data taken into the signal processing device 17 is
By rearranging the data Q as described above in 9, the data sequence as shown in FIG.

C Example 2J It is known that the following properties exist when deriving image information from a subject using an MNR signal. - When there is periodic fluctuation in the ship, the longer the period of change is the same, the fewer artifacts will appear in the image, as is clear from the relationship between FIGS. 1 and 5. In addition, when the signal intensity near the center of a data series is large and the intensity decreases as the distance from the center increases, as in the case of an NMR signal, the data near the center and the peripheral data do not have equal effects on the image.

Considering these points, in the example of FIG. 3, the center of displacement due to respiration occurs twice in one cycle, so in 256 data acquisitions, 164π to 0. π～63
At π, data is collected at the center of displacement. That is, for example, the Gz pulse is -64π as shown in the bottom of Figure 3.
, -128π, 0π, 64π, -63π, -127π,
Data is collected by sequentially applying π, -...-, and the data is rearranged so that the Gz pulse changes sequentially as shown in FIG. 3J. The displacement component due to respiratory vibration included in this rearranged data series is Tp/2 from the displacement center in the interval between 1128π and -65π of the Gz pulse.
The section from -64 to 0 to 63π is the component at the center of displacement, and the section from 64π to 1271t is the component at Tp/4 from the center of displacement.

The smear reduction result corresponding to FIG. 4 in this case is shown in FIG. 6, and parts corresponding to those in FIG. 4 are given the same reference numerals. In this case, the fluctuation component in the rearranged data series becomes a curve 23, with a small displacement at the center of the time series, and opposite displacements before and after that. In the image indicated by diagonal lines, the artifacts 24°25 in Fig. 4 do not exist, and the artifacts near the rise and fall of line 21 are also quite small1, and the image quality is better than in the case of Fig. 5. is improved.

"Example 3" Based on respiration (variations can be predicted in advance, but for unpredictable variations, immediately before collecting NMR signal data, detect the previous changes and change the data collection point to collect data. Later, the data is sorted to obtain image information.

For example, when the magnetic field fluctuates, measure the magnetic field strength in period 2 in Figure 2 immediately before collecting NMR signal data, compare this measured value with the set value, and if the difference is small, it will be close to 0π. A Gz pulse is applied, and if the difference between the measured value and the set value is large, a Gz pulse close to -128π or 127π is applied. In the central part of the data series rearranged in this way, the influence component of magnetic field fluctuations is small, and in the data near the front and rear ends, the influence of magnetic field fluctuations is relatively large. Various algorithms can be considered for importing such data, but
For example, assuming that the probability distribution of the measured value of the magnetic field strength with respect to the set value is a normal distribution, the magnetic field strength and the Gz pulse strength <
A comparison table is prepared in advance, and the intensity of the Gz pulse is determined from the measured value of the magnetic field strength by referring to the comparison table. Number of data collection is 25
The magnetic field measurement values do not have a perfect normal distribution after about 6 times, so if the distribution of the measurement values becomes biased toward the end of data collection, for example, the distribution will shift toward the larger side relative to the set value. If the number increases, you can deal with it flexibly, such as by making the measured value correspond to an example that is symmetrical to the set value.

The data collected in this way is
In a data series rearranged in the order of -128π to 127π by the z pulse, the influence of fluctuations in magnetic field strength is small in the center with 0π as the center, and the influence of fluctuations in magnetic field strength becomes larger closer to both ends. (Moreover, the influence of the variation in magnetic field strength changes gradually along the data series as a whole. In other words, the image quality is improved in the same way as in the second embodiment.

When detecting fluctuations in the magnetic field strength in this way, the magnetic field strength is detected by the magnetic field strength detection means 31 as shown in FIG.
2, and the scan condition determining unit 33 determines the Gz pulse according to the detected fluctuation using the method described above.

In the above description, the present invention was applied to phase encoding, but the present invention can also be applied to frequency encoding. In other words, at each data collection timing,
In the above example, the NMR signal is sampled 256 times.
During the 256 samplings, for example, if there is a possibility of being affected by some fluctuation factor during several milliseconds, the gradient of the magnetic field strength of the X-direction gradient magnetic field G in the above example is changed. After that, data was sampled and imported so that the integral value of the magnetic field strength and the magnetic field application time up to each sampling time became sequential, that is, the gradient of the magnetic field G was kept constant. It is only necessary to perform signal processing to obtain spatial distribution information after rearranging the data into a time data series.In this way, it becomes gentle.

In the above, the NMR signal data was acquired sequentially so that the influence of the fluctuation elements on the rearranged data series was softened, but the fluctuation speed may be increased. If you do this, the artifacts may become separated and you can easily distinguish them.Furthermore, in the above, spatial distribution information was obtained by the two-dimensional Fourier transform method, but spatial distribution information was obtained by the reconstruction method. It's okay.

"Effects of the Invention" As described above, according to the present invention, it is possible to reduce the influence of variable elements when obtaining spatial distribution information from an NMR signal. Moreover, the NMR data collection timing interval T
r can be chosen relatively freely, and the overall data acquisition time can be significantly shorter than in conventional synchronous scanning methods.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an example of an NMR image forming apparatus,
Figure 2 is a diagram showing the pulse sequence of the spinwarb method.
Figure 3 shows variations in the conventional method and the method of this invention, and G
A diagram showing an example of the relationship between the z pulse phase (intensity), the Gz pulse phase indicating the data series of the rearranged row, and the influence component due to fluctuations included in the data series. Figure 4 shows the shape of the object and the Fig. 5 is a diagram showing an example of a simulation of the relationship between an image according to the invention, a Fourier transformation output for frequency encoding, and a fluctuation component included in the output. and the corresponding diagram,
FIG. 6 is a diagram corresponding to FIG. 4 to which the second embodiment of the present invention is applied. Patent applicant Asahi Kasei Industries Co., Ltd. Agent Kusano
Table 2 Figure 74 Note 5 Figure 26 Mouth

Claims (1)

[Claims] (1) Applying a static magnetic field to the subject, and applying to the subject a gradient magnetic field whose generated magnetic field is in the same direction as the static magnetic field and whose strength is gradient in three mutually intersecting directions. Make it possible to discriminate spatial information, apply a high-frequency pulse to the subject in this state, capture the nuclear magnetic resonance signal from the subject, and determine the spatial distribution of the information contained in the nuclear magnetic resonance signal from the captured signal. In this method, the signal is acquired so that the integral value of the strength of the gradient magnetic field with respect to at least one coordinate axis and the magnetic field application time up to the time of signal acquisition does not change sequentially, and then the acquired signal is A method for deriving image information using nuclear magnetic resonance, characterized in that the spatial distribution of the information is obtained by rearranging the information so that the integral value changes sequentially.